Composite membranes capable of selectively permeating one component of a fluid mixture over the remaining components in the mixture generally include a thin layer or coating of a suitable semipermeable membrane material superimposed over a porous substrate. Generally, while the coating affects the separation characteristics of the composite membrane, the primary function of the substrate is to provide support for the coating positioned thereon. Common porous substrates are configured as flat-sheet membranes or as hollow fibers.
In commercial or industrial applications, composite membranes need to operate for extended periods with low incidence of failure. Furthermore, the membranes often must withstand foul or corrosive environments.
One method of preparing composite membranes is to coat a preformed porous substrate, such as a porous hollow fiber, with a dilute solution of a polymer in a solvent, followed by the removal of the solvent by drying. However, it is generally very difficult to produce defect-free thin-film composites with thicknesses of less than 1 micro meter (1 xcexcm) by the solution coating process. Furthermore, it is generally recognized in the art that it is difficult to produce viable (substantially defect-free) high productivity composite membranes by solution coating of substrates having high surface porosity. In particular, in order to produce complete surface coverage, the coating solution must fully wet the substrate. Capillary forces in a fully wetted porous substrate, however, tend to draw the coating solution into the surface pores leading to an effective increase in separation layer thickness and to a decrease in membrane productivity. This is often referred to as occlusion of the pores by the coating solution.
Several processes for making composite membranes are known in the art.
U.S. Pat. No. 4,840,819, to Williams et al., discloses a process in which a dilute solution of permeable polymer is applied to a porous substrate having a controlled amount of liquid incorporated therein.
U.S. Pat. No. 4,806,189, to Kraus et al., discloses a process for producing a composite fluid separation membrane by in situ formation of a separation layer on a porous support wherein the pores of the support are pre-impregnated with a solvent.
U.S. Pat. No. 5,320,754, to Kohn et al., discloses preparation of composite membranes by applying perfluoroethers to the surface of a porous substrate prior to coating with a selective polymeric material.
U.S. Pat. No. 5,213,689, to Kafchinski et al., discloses a method of coating microporous polyolefin hollow fibers by wet spinning or by dry-wet spinning. Polyolefin hollow fibers are coated with SIXEF(trademark)-Durene polyimide containing perfluoro groups from the solvent NMP. The polyolefin hollow fiber is optionally pre-wetted with glycerine prior to coating.
M. Rezac et al. in the Journal of Applied Polymer Science, V46, p.1927(1992), teach preparation of composite membranes from solutions of ultra-high molecular weight polymers. The authors suggest that improved membranes are formed when polymer chain dimensions of the coating material are larger than the surface pores of the porous support.
Several amorphous perfluoropolymers have been used as coating or membrane materials, including perfluoropolymers with high gas permeation characteristics.
U.S. Pat. No. 5,051,114, to Nemser et al., discloses amorphous perfluoro-2,2-dimethyl-1,3-dioxole based polymers that can be used for several separation and gas enrichment applications, including oxygen enrichment of air.
U.S. Pat. No. 4,754,009, to Bowser, discloses a gas permeable material that contains passageways wherein the interior of the passageways is formed by solution coating of perfluoro-2,2-dimethyl-1,3-dioxole.
U.S. Pat. No. 5,876,604, to Nemser et al., discloses the preparation of composite perfluoro-2,2-dimethyl-1,3-dioxole membranes that can be used to add a gas to a liquid or to remove a gas from a liquid. The membranes exhibit resistance to fouling by liquids, and can be utilized for ozonolysis or oxygenation.
U.S. Pat. No. 5,914,154, to Nemser, discloses preparation of non-porous gas permeable membranes by flowing a dilute coating solution of perfluoropolymer through one side of a microporous substrate, until the desired thickness of coating polymer is built up; the solution is then removed and residual solvent is evaporated.
Existing processes for producing composite perfluoropolymer membranes result in relatively thick coating layers, believed to be due, at least in part, to intrusion of the coating layer into the porous support. The resulting composite gas separation perfluoropolymer membranes exhibit relatively low gas permeance.
Therefore, a need exists for high productivity composite membranes and processes for making them in which these problems are eliminated or reduced.
The invention generally is directed to composite membranes, devices including the composite membranes and to methods of producing the composite membranes. The invention also generally is directed to methods of separating a fluid mixture into a fraction enriched in a component and a fraction depleted in the component.
In one embodiment, the invention is directed to a composite membrane which includes a porous asymmetric hollow fiber substrate having a bore side and an outer surface and a perfluorinated polymer coating at the outer surface of the hollow fiber substrate. In another embodiment, the invention is directed to a composite membrane having an oxygen permeance of at least 1500xc3x9710xe2x88x926 cm3(STP)/[(cm2 sec)(cmHg)] and an oxygen/nitrogen gas separation factor of at least 2.1. In yet other embodiments, the composite membranes of the invention are employed in separation devices, also referred to herein as separation modules or separation cartridges.
The invention also is directed to a method of fabricating a composite membrane. The method includes impregnating a porous substrate, such as, for example, an asymmetrical porous hollow fiber substrate, with an impregnation fluid that is immiscible with a perfluorinated solvent. The impregnated substrate is coated with a solution which includes a perfluorinated polymer and the perfluorinated solvent. The method of the invention further includes removing the perfluorinated solvent and the impregnation fluid. Optionally, the impregnation fluid is at least partially removed from the impregnated porous substrate prior to coating.
The invention also is directed to a method for separating a fluid mixture into a fraction enriched in a first component and a fraction depleted in the first component. The method includes contacting the fluid mixture with a composite membrane, whereby the fraction enriched in the first component and the fraction depleted in the first component are generated by preferentially permeating a portion of the fluid mixture through the composite membrane. In a preferred embodiment of the invention, the composite membrane is formed by a process comprising impregnating a porous substrate, such as, for example, an asymmetrical porous hollow fiber substrate, with an impregnation fluid that is immiscible with a perfluorinated solvent; coating the impregnated substrate with a solution which includes a perfluorinated polymer and the perfluorinated solvent; and removing the perfluorinated solvent and the impregnation fluid. In another preferred embodiment, the fluid mixture is air, the fraction enriched in the first component is oxygen-enriched air and the fraction depleted in the first component is nitrogen enriched air. Nitrogen enriched air can be directed into the intake of an internal combustion engine.
The invention has numerous advantages. For example, the coating material can be selected independently of the substrate material and can be tailored towards a specific separation application. In addition, expensive membrane forming materials can be economically utilized as coating materials because only small amounts are required for the formation of the thin coating. Also, the morphology or structure of the coating can be decoupled and optimized independently from the morphology or structure of the substrate.
The process of the invention results in coatings that are both thin and substantially free of defects. The small surface pores characterizing the preferred substrates of the invention can advantageously support thin coating layers, while the larger pore structure underneath enhances the membrane productivity. The composite membranes of the invention exhibit improved gas permeation characteristics as compared to prior art membranes and have a gas separation factor which is close to the measured intrinsic gas separation factor of the coating material. Composite membranes of the invention wherein the coating is at an outer surface of an asymmetric hollow fiber substrate have an increased surface area available for separation processes.
The composite membranes of the invention can withstand foul environments, such as, for example exhaust fumes generated by combustion processes. The separation devices of the invention are characterized by high productivity per unit volume coupled with low permeate and feed side pressure drops and are thus suitable for commercial applications.